It would be highly desirable to provide an air-to-ground (ATG) communication service for providing broadband data, voice and entertainment to the commercial transport industry (e.g., commercial airlines) and general aviation markets in North America and around the world. It would be especially desirable to implement a new ATG network in a manner that is similar to presently existing terrestrial cellular (i.e., wireless) communication networks. This would allow taking advantage of the large amounts of capital that have already been invested in developing cellular technologies, standards and related equipment. The basic idea with a new air-to-ground service would be the same as with other wireless networks. That is, as aircraft fly across North America (or other regions of the world) they are handed off from one base transceiver station (BTS) to another BTS, just as terrestrial cellular networks hand off cellular devices (handsets, PDAs, etc.) when such devices are mobile.
One important difference is that ATG systems use one transceiver having an antenna mounted on the undercarriage of the aircraft to communicate with the terrestrial BTS. Presently, the Federal Communications Commission (FCC) has allocated only a single 1.25 MHz channel (in each direction) for ATG use. This creates a significant problem. There simply is insufficient communication capacity in a single ATG channel to provide broadband service to the expected market of 10,000 or more aircraft using the exact communication method and apparatus used for standard terrestrial cellular communication. For example, if one was to take a standard cell phone handset and project its omnidirectional radiation pattern outside the skin of the aircraft, and allowed the signals to communicate with the terrestrial cellular network, such a system would likely work in a satisfactory manner, but there would be insufficient capacity in such a network to support cellular users on 10,000 or more aircraft.
The above capacity problem comes about because a typical cell phone antenna is a monopole element that has an omnidirectional gain pattern in the plane perpendicular to the antenna element. This causes transmit power from the antenna to radiate in all directions, thus causing interference into all BTS sites within the radio horizon of its transmissions (a 250 mile (402.5 km) radius for aircraft flying at 35,000 ft (10,616 m) cruise altitude. All cellular networks, and especially those using code division multiple access (CDMA) technology, are limited in their communication capacity by the interference produced by the radiation from the mobile to cellular devices used to access the networks.
A well known method for reducing interference on wireless networks is using directional antennas instead of the omnidirectional antennas used on mobile cellular phones. Directional antennas transmit a directional beam from the mobile cellular phones towards the intended target (i.e., the serving BTS) and away from adjacent BTS sites. This method can increase the network capacity by several fold, but it is impractical for most personal cell phones because the directional antennas are typically physically large, and certainly not of a convenient size for individuals to carry and use on a handheld cellular phone. However, directional antennas can easily be accommodated on most mobile platforms (e.g., cars, trucks, boats, trains, buses, aircraft and rotorcraft).
Accordingly, a fundamental problem is how to implement commercial off-the-shelf (COTS) cellular technology, designed to operate with omnidirectional antennas, to function properly with directional antennas. A closely related technical problem is how to implement hand offs of mobile cellular phones between BTS sites using standard methods and protocols. In particular, the 3rd generation cellular standards (CDMA2000 and UMTS) both use a method called “soft handoff” to achieve reliable handoffs with very low probability of dropped calls. To be fully compatible with these standards, any new ATG service must support soft handoffs. A specific technical issue, however, is that performing a soft handoff requires that the mobile cellular terminal (i.e., cell phone) establishes communication with one BTS before breaking communication with another BTS. This is termed a “make before break” protocol. The use of a conventional antenna to look in only one direction at a time, however, presents problems in implementing a “make before break” soft handoff. Specifically, conventional directional antennas have only a single antenna beam or lobe. If the mobile platform, for example a commercial aircraft, wants to handoff from a BTS behind it (i.e., a BTS site that the aircraft has just flown past) in order to establish communication with another BTS that the aircraft is approaching, it must break the connection with the existing BTS before making a new connection with the new BTS that it is approaching (i.e., a “break before make” handoff). A “break before make” handoff is also known as a “hard handoff.” As mentioned previously, this is not as reliable a handoff method as the “make before break” handoff, although it is used presently in second generation TDMA cellular systems, and is also used under unusual circumstances (e.g., channel handoff) in 3rd generation cellular systems.
Thus, in order to implement soft handoffs in an ATG system implemented with using a high speed mobile platform such as a commercial aircraft, the fundamental problem remaining is how to achieve soft handoffs using directional antennas.